Unit 1.1 Topic 5

Many organelles are membrane-bound. They form separate compartments within the cell.

The Nucleus- The nucleus is the largest organelle. When stained, it shows darkened patches known as chromatin. It is surrounded by the nuclear envelope. This is the structure made of two membranes with fluid between them. A lot of holes, called nuclear pores, go right through the envelope. These holes are large enough for relatively large molecules to pass through. There is a dense, spherical structure, called the nucleolus, inside the nucleus. The nucleus:

Houses nearly all the cell's genetic material. the chromatin consist of DNA and proteins. It has instructions for making proteins. Some of these proteins regulate the cell's activities.

When cells divide, chromatin condenses into visible chromosomes.

The nucleolus makes RNA and ribosome. These pass into the cytoplasm and proteins are assembled at them.

Endoplasmic reticulum (ER)- ER consist of a series of flattened membrane-bound sacs called cisternae. Rough endoplasmic reticulum is studded with ribosomes. Smooth endoplasmic reticulum does not have ribosomes.

Rough ER transports proteins that were made on the attached ribosomes. Some of these proteins may be secreted from the cell. Some will be placed on the cell surface membrane.